Reactions between gases and liquids are mostly carried out as countercurrent operations. If the rates of liquid and gas are relatively high, towers are used, which contain packing elements and in which the liquid trickles in a countercurrent to the gas. If the rate of liquid flow is low relative to the rate of gas flow, bubble cap trays or similar internal fixtures are employed. It is more difficult to optimize such equipment for reactions in which a gas is formed from components of the liquid. In such cases, the rate at which the gas is formed must be taken into account in the structural design of the reactor so that the volume ratio of gas to liquid must be taken into account as well as the rate of the gas-producing chemical reaction. In cases in which the liquid to be reacted is nevertheless to be conducted in a countercurrent to the gas, cascades comprising a plurality of reactor vessels have been used and the vessels of the cascade have been laterally offset or vertically aligned (German patent specification No. 924,689). If the several vessels or reactor chambers are vertically aligned, they may be interconnected, e.g., by immersed overflow ducts for the liquid or by immersed gas ducts, each of which basically corresponds to a large bubble cap of a bubble cap tray (Published German Application No. 31 18 795).
A typical example of such a gas-producing chemical reaction is the production of chlorine dioxide, e.g. by the action of hydrochloric acid on sodium chlorate in a process in which a purging or diluent gas is conducted in a countercurrent.
The formation of ClO.sub.2 may be the result of two reactions, both of which occur in practice. Reaction 1.2 should be suppressed as far as possible. EQU 1.1 NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +NaCl+H.sub.2 O EQU 1.2 NaClO.sub.3 +6HCl.fwdarw.3Cl.sub.2 +NaCl+3H.sub.2 O
In that process of producing ClO.sub.2, the following requirements must be met:
(a) The ratio of NaClO.sub.3 to HCl must be high so that reaction 1.1 will be promoted (see Swindell Pulping Conference, pages 195 et seq., 1984).
(b) The concentrations of NaClO.sub.3 and HCl must be as high as possible so that the formation of ClO.sub.2 is accelerated owing to the fact that the rate at which ClO.sub.2 is formed is directly proportional to the product of the multiplication of the HCl concentration and NaClO.sub.3 concentration.
A reaction of the entire quantity of acid and the entire quantity of chlorate will take some hours.
It is known that the actual controlling factor is the H.sup.+ ion concentration rather than the concentration of hydrochloric acid. It has been attempted to control the H.sup.+ ion concentration by an addition of buffer solutions (see U.S. Pat. No. 4,086,328).
Because the Na.sub.2 Cr.sub.2 O.sub.7 acts as a buffer, the higher efficiencies stated by Swindell will be obtained if said substance is employed. As a result, any acid can be used, on principle, in combination with a reducing agent in known manner to produce ClO.sub.2 from NaClO.sub.3, e.g., NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +Na.sub.2 SO.sub.4 +H.sub.2 O or 2 NaClO.sub.3 +SO.sub.2 .fwdarw.2 ClO.sub.2 +Na.sub.2 SO.sub.4
For these two reactions, a supply of H.sub.2 SO.sub.4 at one point will be required.
Because chlorine dioxide is a gaseous chemical compound which in excessively high concentrations decomposes explosively, the partial pressure must be reduced in order to avoid explosions. This can be accomplished in that a subatmospheric pressure is maintained. Alternatively, the gas may be diluted with another gas or both measures may be combined. The ClO.sub.2 gas is blown within seconds or a few minutes.
Owing to the high acid concentration required and to the need for blowing out the mixed ClO.sub.2 --Cl.sub.2 gases, the process is ideally carried out as a countercurrent operation.